I'd be amused if that was the joke -- that everyone listed actually had a different natural hair color and thus were "Hollywood blonds." Certainly more than a few in there who aren't natural blonds, but probably quite a lot who are.

Several(many?) of the most prototypical "blondes" have in fact dyed their hair - Marilyn, Jean Harlow, Madonna. According to some academic commentators, being "blonde" is more a state of mind than an accident of birth. If you think of it that way, it makes some sort of sense for the top "Hollywood Blondes" to not actually be blonde.